On Aging: The bigger they are, the older they grow

Updated 10:55 am, Monday, April 9, 2012

Sometimes we recognize patterns in the world so intuitively that we don't appreciate their profundity. Throw a mouse off a 10-story building, for instance, and it will walk away unhurt (depending somewhat on whether it lands on concrete or dirt). Do the same for a dog, it will be killed. A horse will splat. (Now don't try this experiment at home. Take my word for it.)

We don't need to know the laws of physics to anticipate the results, but if we thought about this pattern for a few minutes it might tell us something about the laws of gravity, about the nature of air resistance and about how the world is a very different place depending on what size you are. You won't find horse-size animals scrambling around treetops, but you will find lots of mice-size ones.

Size also affects how fast things age. Again, the pattern is familiar. That hamster you had when you were a child, as well as its babies, their babies, their babies and several more generations, will have long since hopped away to hamster heaven by the time you became an adult. Your childhood dog, on the other hand, may have grown a bit slow and slightly decrepit but was still likely to be alive when you came home from college or boot camp.

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Had you owned a pet elephant, on the other hand (again, don't try this at home), it would not have changed much between your childhood and the time you bought your first legal drink. Big species age more slowly and live longer than small species.

Does this mean that really big species, like whales, live even longer than we do? This is not an easy question to answer. Most of what we know about the longevity of other species comes from pets or zoos and rely on dependable birth records. Except for a few small whales such as SeaWorld's Shamus, whales live in the wild, and wild whales' birth certificates are not as accurate as we might like.

But biologists have other clever ways of estimating an animal's age. In the case of the bowhead whale — the second-largest whale on the planet — researchers estimated their age by analyzing certain proteins in the eyes of animals killed during Eskimo hunts.

The surprising results: Nearly 10 percent of the animals studied were estimated to be more than 150 years old; the oldest was estimated to have been killed at the extremely ripe age of 211 years.

To put this in perspective, only about one in 5,000 Americans today lives to be 100 years, and the current oldest person in the world is about 114 years old.

Other whale researchers were skeptical about these age estimates. But over the past few years, a variety of traditional stone and ivory harpoon tips have been recovered from the bodies of recently deceased whales. Comparing these implements to collections in the Smithsonian Institution, suggests they could have been as old as a few hundred years and no more recent than about 120 years. Suddenly, a 200-year-old whale did not seem so far-fetched.

But how does such extreme longevity fit with the bowhead whale's size? Quite well, it turns out. With a little mathematical tinkering, you can draw a straight line through a graph of animal size versus animal longevity, and most species hug that line quite closely. If you are a mammal weighing about 200,000 pounds like the bowhead whale, this line predicts you should live about 200 years. The pattern persists.

Wait a second — don't humans live a lot longer than expected for a species our size? That's another interesting question — and the topic of my next column.